1. Field of the Invention.
The present invention relates to filtration media, and, particularly, to antistatic, electrically conductive filtration material.
2. Description of Prior References.
Control of static electricity can be of great importance in many industrial settings where an uncontrolled electrostatic discharge (ESD) or spark can result in serious damage. For example, static discharges can bring about the destruction of integrated circuits during some stages of their manufacture. In explosive environments, such as in grain elevators, or in flammable environments, such as on oil drilling rigs, in refineries, and in solvent-based processes, a static discharge can be extremely dangerous and must be prevented in order to safeguard life and property.
Organic polymeric textile materials used in these settings can be the source of static discharges due to the normally insulative nature of the materials. Further, such material may have a high value of specific resistance, typically on the order of 10.sup.12 ohm-cm or higher, unless the materials are altered to prevent build up of electrical charges on their surfaces by permitting charges found on their surfaces to drain in a controlled manner. A particularly preferred filtration media is expanded PTFE, such as that disclosed in U.S. Pat. No. 3,953,566 to Gore. While this material supplies a very good filtration efficiency, it is electrically resistant and untreated will not dissipate static electricity.
To control static electrical charges found in textile materials, electrical conductivity of organic polymeric textile material may be increased through application of antistatic finishes to the textile material or through introduction of at least partially conductive fibers into the textile material. Other means for controlling static electric charges include external devices to carry electrical charges found on the textile material to ground (e.g., grounding straps or wire).
One method of discharging static charges is to apply an antistatic finish to organic polymeric textile materials. This may be performed either when the organic polymeric textile material is in fiber form or in fabric form. These finishes typically increase ionic conductivity of the surface on which they are applied thereby promoting static dissipation. However, these finishes are typically not as durable as the polymeric textile materials on which they are applied. Cleansing or merely using the organic polymeric textile material can remove these finishes from the fabric surface, resulting in a loss of the material's ability to dissipate static electric charges.
Another approach is to apply a coating of metals or of conductive carbon to the outside surface of fibers used in producing organic polymeric textile material. However, if the coating used is not as flexible as the fiber on which it is placed, flexing of the fiber may cause cracks in the coating that may interrupt or destroy the conductive pathway formed by the coating.
Still another strategy to drain off static charges is to produce textile materials incorporating conductive fibers into a matrix of nonwoven filtration media. Examples of conductive fibers include carbon fibers, metal fibers, or filled expanded polytetrafluoroethylene (PTFE) fibers, such as that disclosed in U.S. Pat. No. 5,229,200 to Sassa.
While the above materials function well under some applications, they do not always address all requirements for electrostatic dissipation in all applications. U.S. Pat. No. 5,229,200 to Sassa et al., employs a filter media comprised of a static dissipative nonwoven textile material (support layer) which is laminated to an electrically insulative porous polymeric membrane (filtration layer), especially an expanded polytetrafluoroethylene (ePTFE) membrane. This media can meet some of the requirements of high filtration efficiencies and static dissipation. However, the insulative porous polymeric membrane limits the ability of the filter media to conduct electrical charges, thus restricting this media's use in extremely spark-sensitive environments. Additionally, some industries have developed standards for filtration media that require a surface resistance of a minimum value. The insulating porous polymeric membrane used in existing laminated filter media will not pass many of these specifications due to the high resistivity of the membrane.
Accordingly, it is a purpose of the present invention to provide a conductive filtration media that has both a high filtration efficiency and effective static dissipation properties.
It is another purpose of the present invention to provide a conductive filtration media that has the high filtration efficiency of expanded PTFE while having sufficient electrical dissipation properties to allow use in demanding explosive environments.
These and other purposes of the present invention will become evident through review of the following specification.